When it comes to scheduling and setup, more serious than average for a leisure activity. We set time aside in our busy lives to make D&D night possible, so when somebody texts or e-mails everybody to say, "sorry, I don't feel like it tonight," that irritates me. On the other hand, it doesn't trump work, family or life obligations. Though we did have a couple people call in sick to work for our last session so we could do an all-day going away game for one of our players who moved away.
As far as in-game — I find that my adventures are usually more serious when I plan them than when I actually run them at the table. It's easier to go broad with a voice or a characterization at the table, especially when you're fleshing out details on the spot and you've had a few beers. Side characters who get unexpected spotlight time with the PCs tend to get a lot of quirks thrown on in a hurry. I think the humor helps to keep people engaged, and makes things easier for me, but it probably does make some of the deeper immersion, particularly in more horror settings, impossible.
As a role-player, I am particularly bad about this and have been trying to work harder at being a little more subtle and consistent with my player characters. Not that I'll ever stop doing the accents or start playing brooding loner rangers, but having my character develop a new strong phobia or obsession every session is probably tedious for everybody else.
